As the semiconductor industry shrinks the size of features on integrated circuits, wafer fabs require higher-resolution techniques for inspecting silicon wafers and photomasks. Electron-beam tools for metrology, offline inspection, and even online inspection have reached the marketplace. These systems are conventional scanning electron microscopes in which an incident beam of high-energy electrons strikes the wafer, causing secondary and other electrons to leave the surface. The electrons are detected, and the systems create an image of the wafer surface.
Scanning electron microscopes in the prior art have attempted to solve the problem of charge control; i.e., preventing the accumulation of a positive charge on the surface of the wafer as secondary electrons leave the surface of the material. This is a difficult problem, and many approaches have been attempted.